Structural panels requiring light weight but high rigidity, notably aircraft wings, may be made of a construction wherein honeycomb or other cellular type material acts as a substrate which is bonded at its upstanding edges to a face sheet. It is important in these structural panels that a secure adhering bond be maintained between the face sheet and the substrate substantially over the entire area of the panel. This invention is related to an apparatus for the non-destructive testing of the bonds between the substrate and the face sheet.
Non-destructive testing of the bonds has been accomplished by other known means, including the means disclosed in my U.S. Pat. No. 4,043,179 in which an internally evacuated transducer is arranged to glide easily over the surface being scanned while maintaining a constant or pulsating vacuum. When the apparatus of my prior patent is drawn over an area where the face sheet is not properly bonded to the substrate, the face sheet is drawn by the vacuum into a detection chamber where detection is accomplished by the displacement of core rods in one or more linear variable differential transformers (LVDT).
As my prior invention was an improvement over an earlier suction-type device, so also is this invention an improvement on my prior apparatus. The present invention takes advantage of the principle of creating an air cushion beneath the apparatus by high velocity air jets, which provides a virtually frictionless bearing surface so that the detection apparatus may be easily and rapidly skimmed over the face sheet. Advantage is taken of the Bernoulli principle of moving fluids in order to use the air jets producing the air cushion to simultaneously produce a vacuum to displace the face sheet in areas where the sheet is not properly bonded to the substrate. Advantage is also taken of the principles of capacitance gauges in order to detect when the substrate is displaced toward the testing apparatus by the vacuum.
It should be apparent at this point, and will become clear upon reading the disclosure, that there are many significant advantages in the present invention. The examining surface of the present apparatus does not come in contact with the face sheet. This eliminates the need for hermetic seals as required by the earliest detection devices of this type, and also removes the necessity of a separate vacuum chamber and counteracting compressed air portion separated by seals, as in my previous apparatus. Considerable weight is eliminated by not having a requirement for separate evacuating means and air compressing means. In addition, the capacitance gauge sensing eliminates considerable weight as compared to the LVDT's and provides sensing of smaller displacements of the face sheet.
These and other advantages will become apparent upon reading the disclosure which follows.